System and method for synchronizing data transmission from multiple wireless base transceiver stations to a subscriber unit

ABSTRACT

The invention includes an apparatus and a method for transmitting sub-protocol data units from a plurality of base transceiver stations to a subscriber unit. The method includes estimating time delays required for transferring the sub-protocol data units between a scheduler unit and each of the base transceiver stations. The method further includes the scheduler unit generating a schedule of time slots and frequency blocks in which the sub-protocol data units are to be transmitted from the base transceiver stations to the subscriber unit. The time delays are used to generate the schedule. The time delays can be used to generate a look ahead schedule that compensates for the timing delays of the sub-protocol data units from the scheduler unit to the base transceiver stations. The sub-protocol data units are wirelessly transmitted from the base transceiver stations to the subscriber unit according to the schedule. The time delays can be estimated by time-stamping sub-protocol data units before sub-protocol data units are transferred from the scheduler unit to the base transceiver stations, and estimating the time delays by comparing the times the sub-protocol data units are actually received by the base transceiver stations with the time-stamping.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application is a Continuation of application Ser. No.09/729,886, filed Dec. 4, 2000 now U.S. Pat. No. 6,862,272 which is aContinuation-in-Part of U.S. patent application Ser. No. 09/708,170,filed Nov. 7, 2000 now U.S. Pat. No. 6,567,387.

FIELD OF THE INVENTION

The invention relates generally to wireless communications. Moreparticularly, the invention relates to synchronizing transmission ofdata between multiple base transceiver stations and subscriber units,providing spatial multiplexing and communication diversity.

BACKGROUND OF THE INVENTION

Wireless communication systems commonly include information carryingmodulated carrier signals that are wirelessly transmitted from atransmission source (for example, a base transceiver station) to one ormore receivers (for example, subscriber units) within an area or region.

Spatial Multiplexing

Spatial multiplexing is a transmission technology that exploits multipleantennae at both the base transceiver station and at the subscriberunits to increase the bit rate in a wireless radio link with noadditional power or bandwidth consumption. Under certain conditions,spatial multiplexing offers a linear increase in spectrum efficiencywith the number of antennae. For example, if three antennae are used atthe transmitter (base transceiver station) and the receiver (subscriberunit), the stream of possibly coded information symbols is split intothree independent substreams. These substreams occupy the same channelof a multiple access protocol. Possible same channel multiple accessprotocols include a same time slot in a time-division multiple accessprotocol, a same frequency slot in frequency-division multiple accessprotocol, a same code sequence in code-division multiple access protocolor a same spatial target location in space-division multiple accessprotocol. The substreams are applied separately to the transmit antennaeand transmitted through a radio channel. Due to the presence of variousscattering objects in the environment, each signal experiences multipathpropagation.

The composite signals resulting from the transmission are finallycaptured by an array of receiving antennae with random phase andamplitudes. At the receiver array, a spatial signature of each of thereceived signals is estimated. Based on the spatial signatures, a signalprocessing technique is applied to separate the signals, recovering theoriginal substreams.

FIG. 1 shows three transmitter antenna arrays 110, 120, 130 thattransmit data symbols to a receiver antenna array 140. Each transmitterantenna array includes spatially separate antennae. A receiver connectedto the receiver antenna array 140 separates the received signals.

FIG. 2 shows modulated carrier signals traveling from a transmitter 210to a receiver 220 following many different (multiple) transmissionpaths.

Multipath can include a composition of a primary signal plus duplicateor echoed images caused by reflections of signals off objects betweenthe transmitter and receiver. The receiver may receive the primarysignal sent by the transmitter, but also receives secondary signals thatare reflected off objects located in the signal path. The reflectedsignals arrive at the receiver later than the primary signal. Due tothis misalignment, the multipath signals can cause intersymbolinterference or distortion of the received signal.

The actual received signal can include a combination of a primary andseveral reflected signals. Because the distance traveled by the originalsignal is shorter than the reflected signals, the signals are receivedat different times. The time difference between the first received andthe last received signal is called the delay spread and can be as greatas several micro-seconds.

The multiple paths traveled by the modulated carrier signal typicallyresults in fading of the modulated carrier signal. Fading causes themodulated carrier signal to attenuate in amplitude when multiple pathssubtractively combine.

Communication Diversity

Antenna diversity is a technique used in multiple antenna-basedcommunication system to reduce the effects of multi-path fading. Antennadiversity can be obtained by providing a transmitter and/or a receiverwith two or more antennae. These multiple antennae imply multiplechannels that suffer from fading in a statistically independent manner.Therefore, when one channel is fading due to the destructive effects ofmulti-path interference, another of the channels is unlikely to besuffering from fading simultaneously. By virtue of the redundancyprovided by these independent channels, a receiver can often reduce thedetrimental effects of fading.

An individual transmission link exists between each individual basetransceiver station antenna and a subscriber unit in communication withthe base transceiver station. The previously described spatialmultiplexing and communication diversity require multiple antennas toeach have transmission links with a single subscriber unit. Optimally,the base transceiver station can schedule data transmission according tothe transmission link quality.

It is desirable to have an apparatus and method that provides schedulingof transmission of data blocks between multiple base stationtransceivers and receivers (subscriber) units. It is desirable that thescheduling be adaptive to the quality of transmission links between thebase station transceivers and the receivers (subscriber) units. It isadditionally desirable that the apparatus and method allow for spatialmultiplexing and communication diversity through the multiple basestation transceivers.

SUMMARY OF THE INVENTION

As shown in the drawings for purposes of illustration, the invention isembodied in an apparatus and a method for scheduling wirelesstransmission of data blocks between multiple base transceiver stationsand multiple receiver (subscriber) units. The scheduling accounts fortime delays that exist between a scheduler unit and the base transceiverstations. The scheduling can be based on the quality of a transmissionlink between the base transceiver stations and the receiver units, theamount of data requested by the receiver units, and/or the type of datarequested by the receiver units. The scheduling generally includesassigning frequency blocks and time slots to each of the receiver unitsfor receiving or transmitting data blocks. The transmission schedulingallows for spatial multiplexing and communication diversity throughspatially separate base station transceivers.

A first embodiment of the invention includes a method of transmittingsub-protocol data units from a plurality of base transceiver stations toa subscriber unit. The method includes estimating time delays requiredfor transferring the sub-protocol data units between a scheduler unitand each of the base transceiver stations. The method further includesthe scheduler unit generating a schedule of time slots and frequencyblocks in which the sub-protocol data units are to be transmitted fromthe base transceiver stations to the subscriber unit. This embodimentcan include the time delays being used to generate the schedule.

A second embodiment of the invention is similar to the first embodiment.The second embodiment further includes the time delays being used togenerate the schedule by using the time delays to project the timing ofwhen the sub-protocol data units are to be wirelessly transmitted fromthe base transceiver stations.

A third embodiment is similar to the second embodiment. The thirdembodiment includes a the time delays being used to generate a lookahead schedule that compensates for the timing delays of transferringthe sub-protocol data units from the scheduler unit to the basetransceiver stations.

A fourth embodiment is similar to the first embodiment. The fourthembodiment includes wirelessly transmitting the sub-protocol data unitsfrom the base transceiver stations to the subscriber unit according tothe schedule.

A fifth embodiment is similar to the first embodiment. The fifthembodiment includes the estimating time delays required for transferringthe sub-protocol data units between the scheduler unit and the basetransceiver stations by time-stamping sub-protocol data units beforesub-protocol data units are transferred from the scheduler unit to thebase transceiver stations, and estimating the time delays by comparingthe times the sub-protocol data units are actually received by the basetransceiver stations with the times of the time-stamping.

A sixth embodiment is similar to the first embodiment. The sixthembodiment includes the scheduler receiving standard protocol data unitsfrom a network and sub-dividing the standard protocol data units formingthe sub-protocol data units.

A seventh embodiment is similar to the first embodiment. The seventhembodiment includes synchronizing the base transceiver stations to acommon reference clock. The synchronization can include receiving aglobal positioning satellite (GPS) signal, and generating the commonreference clock from the GPS signal.

A eighth embodiment is similar to the first embodiment. The eighthembodiment includes the sub-protocol data units being transmittedbetween the base transceiver stations and the subscriber unit in datablocks, the data blocks being defined by a frequency block and timeslot. Generally, the scheduler unit generates a map that determines whenthe data blocks are transmitted the base transceiver stations and thesubscriber unit.

An ninth embodiment includes a cellular wireless communication system.The communication system includes a scheduler unit. The scheduler unitreceives the protocol data units from a network and sub-dividing theprotocol data units into sub-protocol data units. A plurality of basetransceiver stations receive the sub-protocol data units, and wirelesslytransmitting the sub-protocol data units to a subscriber unit. Timedelays for transferring the sub-protocol data units from the schedulerunit to the base transceiver stations are estimated. The scheduler unitdetermines a schedule protocol for transmission of the sub-protocol dataunits by the plurality of base transceiver stations. The scheduleaccounts for the time delays.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art wireless system that includes spatiallyseparate transmitters.

FIG. 2 shows a prior art wireless system that includes multiple pathsfrom a system transmitter to a system receiver.

FIG. 3 shows an embodiment of the invention.

FIG. 4 shows another embodiment of the invention.

FIG. 5 show the time delays between the base station controller and thebase transceiver stations of FIG. 3.

FIG. 6 shows the time delays between the home base transceiver stationand the base transceiver stations of FIG. 4.

FIG. 7 shows an example format of a sub-protocol data unit.

FIG. 8 shows how the example sub-protocol data unit of FIG. 7 can beencapsulated within an asynchronous transmission mode (ATM) networktransmission unit.

FIG. 9 shows how the example sub-protocol data unit of FIG. 7 can beencapsulated within an internet protocol (IP) network transmission unit.

FIG. 10A shows a flow chart of steps included within an embodiment ofthe invention.

FIG. 10B show another flow chart of steps included within anotherembodiment of the invention.

FIG. 11A shows a set of service flow requests that indicate demands fordata by subscriber units.

FIG. 11B shows a set of estimated service flow buffer sizes thatindicate demands for up link data by subscriber units.

FIG. 12 shows a frequency spectrum of OFDM sub-carrier signals.

FIG. 13A shows a frame structure depicting blocks of transmission datadefined by transmission time and transmission frequency.

FIG. 13B shows a frame structure that includes an up link maptransmitted at one frequency band, and a down link map transmitted atanother frequency band.

FIG. 13C shows a frame structure that include an up link map transmittedat a first time, and a down link map transmitted at a second time.

FIG. 14 shows an example of a service flow table.

FIG. 15 shows a flow chart of steps included within an embodiment of ascheduler according to the invention.

FIG. 16 depicts several modes of block transmission according to theinvention.

FIG. 17 shows a structure of a map message that is sent once per frame.

DETAILED DESCRIPTION

As shown in the drawings for purposes of illustration, the invention isembodied in an apparatus and a method for scheduling wirelesstransmission of data blocks between multiple base transceiver stationsand multiple receiver (subscriber) units. The scheduling accounts fortime delays that exist between a scheduler unit and the base transceiverstations. The scheduling can be based on the quality of a transmissionlink between the base transceiver stations and the receiver units, theamount of data requested by the receiver units, and/or the type of datarequested by the receiver units. The scheduling generally includesassigning frequency blocks and time slots to each of the receiver unitsfor receiving or transmitting data blocks. The transmission schedulingallows for spatial multiplexing and communication diversity throughspatially separate base station transceivers.

FIG. 3 shows an embodiment of the invention. The embodiment includes abase station controller 310 receiving standard protocol data units(PDUs). The PDUs are divided into smaller sub-protocol data units thatare stored in memory in the base station controller 310. The basestation controller 310 is connected to multiple base transceiverstations 330, 350, 370. The base station controller 310 includes ascheduler 316. The scheduler 316 generates a map that designates timeslots and frequency block in which the sub-protocol data units are to betransmitted from the base transceiver stations 330, 350, 370 to receiver(subscriber) units 397, 399 (down link), and time slots and frequencyblocks in which other sub-protocol data units are to be transmitted fromthe receiver (subscriber) units 397, 399 to the base transceiverstations 330, 350, 370 (up link).

The data interface connections 355 between the base station controller310 and the multiple base transceiver stations 330, 350, 370, aregenerally implemented with standard networking protocols because theseprotocol have been well established and adopted. The standard networkingprotocols can be, for example, asynchronous transmission mode (ATM) orinternet protocol (IP) interconnection networks. Other types of standardnetworking protocols can be used. The sub-protocol data units are notdirectly adaptable for transmission over ATM or IP networks. Therefore,the sub-protocol data units must be encapsulated within an ATM or IPpacket switched data unit. The encapsulation process will be discussedlater.

A media access control (MAC) adaptation unit 312 within the base stationcontroller 310 receives the protocol data units from a standard computernetwork. The protocol data units can be ethernet frames, ATM cells or IPpackets. The MAC adaptation unit 312 divides the protocol data unitsinto smaller sub-protocol data units that are more adaptable fortransmission within wireless communication systems. Smaller sub-protocoldata units facilitate error recovery through retransmission.

The digital circuitry required to divide or break large groups of datainto smaller groups of data is well known in the art of digital circuitdesign.

The sub-protocol data units are stored within sub-protocol data unitbuffers 314 of the base station controller 310. The sub-protocol dataunit buffers 314 provide easy access to the sub-protocol data unitsaccording to a transmission schedule.

A scheduler 316 generates a map or schedule of transmission of thesub-protocol data. This includes when and at what frequency rangesub-protocol data units are to be received by the receiver (subscriber)unit 397, 399, and when and at what frequency range the receiver(subscriber) units 397, 399, transmit sub-protocol data units back tothe base station transceivers 330, 350, 370. The map is transmitted tothe receiver (subscriber) units 397, 399, so that each receiver(subscriber) unit knows when to receive and transmit sub-protocol units.A map is transmitted once per a unit of time that is generally referredto as a frame. The time duration of the frame is variable.

The scheduler 316 receives information regarding the quality oftransmission links between the base station transceivers 330, 350, 370and the receiver (subscriber) units 397, 399. The quality of the linkscan be used to determine whether the transmission of data to aparticular receiver should include spatial multiplexing or communicationdiversity. Additionally, the scheduler 316 receives data requests fromthe receiver (subscriber) units. The data requests include informationregarding the size of the data request, and the data type of the datarequest. The scheduler includes the link quality information, the datasize, and the data type for generating the schedule. A detaileddiscussion of an implementation of the scheduler will follow.

The scheduler 316 accesses the sub-protocol data units within thesub-protocol data buffers 314. A predetermined number of sub-protocoldata units are retrieved by the scheduler 316 and ordered within framesof framing units 332, 352, 372 within the base transceiver stations 330,350, 370. A map of the schedule is include within every frame for thepurpose of indicating to each receiver unit when and at what frequencydata blocks requested by the receiver unit will be transmitted, and whenand at what frequency data blocks can be transmitted from the receiverunit. The frame includes a predetermined number of sub-protocol dataunits as will be described later. Implementations of the framing units332, 352, 372 will be discussed later.

The framed sub-protocol data units are received by coding, diversityprocessing, multi-carrier modulation units 334, 354, 374. The codingwithin the units 334, 354, 374 will be discussed later. The units 334,354, 374 can include diversity processing of the sub-protocol units.Diversity communications and processing is well known in the field ofcommunications.

Multi-carrier modulator units 334, 354, 374 each generate a plurality ofmultiple-carrier modulated signals. Each multi-carrier modulator 334,354, 374 receives a processed (coding and/or diversity processing)sub-protocol data unit stream and generates a multiple-carrier modulatedsignal based on the corresponding processed sub-protocol data unitstream. The multiple-carrier modulated signals are frequencyup-converted and amplified as is well known in the art of communicationsystems.

An output of a first multi-carrier modulator 334 is connected to a firsttransmit antenna 384. An output of a second multi-carrier modulator 354is connected to a second transmit antenna 382. An output of a thirdmulti-carrier modulator 374 is connected to a third transmit antenna386. The first transmit antenna 384, the second transmit antenna 382,and the third transmit antenna 386 can be located within an antennaarray at a single base station. Alternatively, the first transmitantenna 384, the second transmit antenna 382, and the third transmitantenna 386 can each be located at separate base stations. The firsttransmit antenna 384, the second transmit antenna 382, and the thirdtransmit antenna 386 can have different polarization states. Circuitryassociated with the transmitter chains can be separately located withthe antennas 384, 382, 386.

The embodiment of FIG. 3 includes three transmit base transceiverstations. It is to be understood that the invention can include two ormore transmit base transceiver stations. The additional antennas can bedriven by additional multi-carrier modulators that each include separatecorresponding processed sub-protocol data unit streams.

The embodiment of FIG. 3 includes subscriber units 397, 399. Thesubscribers units 397, 399 can include multiple spatially separatesubscriber antennae.

Multiple transmitter antennae and/or multiple receiver antennae allowthe wireless communication system to include spatial multiplexing andcommunication diversity. As described earlier, spatial multiplexing andcommunication diversity can improve the capacity of the communicationsystem and reduce the effects of fading and multi-path resulting inincreased capacity.

Spatial multiplexing and diversity require sub-protocol data unitstransmitted from separate base stations and to be received by commonreceiver (subscriber) units to be precisely synchronized in time. Thatis, if a receiver (subscriber) unit is to receive sub-protocol dataunits from separate base transceiver stations, in a same frequency blockand time slot, the base transceiver stations must be synchronized, andtime delays between the base station controller and the base transceiverstations must be known.

Timing and Synchronization of the Base Transceiver Stations

The embodiments of the invention include transmitting information frommultiple base transceiver stations that are physically separated. Aspreviously stated, the scheduler 316 generates a map that depicts timeslots and frequency block in which the sub-protocol data units are to betransmitted from the base transceiver stations 330, 350, 370 to receiver(subscriber) units 397, 399, and time slots and frequency blocks inwhich other sub-protocol data units are to be transmitted from thereceiver (subscriber) units 397, 399 to the base transceiver stations330, 350, 370. However, because the base transceiver stations aretypically located at different locations than the base stationcontroller, a time delay generally exists between the base stationcontroller and the base transceiver stations. That is, when sub-protocoldata units are accessed from the sub-protocol data unit buffers fortransmission from a base transceiver station, a delay will occur due tothe time required to transfer the sub-protocol data units to the basetransceiver station.

In order for a multiple antenna system to properly operate, sub-protocoldata units must be simultaneously transmitted from multiple basetransceiver stations. Additionally, the scheduler must be able todetermine which sub-protocol data units are simultaneously transmitted.The above-described delay of the sub-protocol data units generallyrequires the base transceiver stations and the base transceivercontroller to be synchronized to a common reference clock. Additionally,the scheduler generally specifies the transmission time of eachsub-protocol data units.

The propagation and transmission delays between the base stationcontroller to the base transceiver stations, are typically variable. Tocompensate for the variable delay, the base station controller caninclude “look-ahead” scheduling. That is, the scheduler computes aschedule for a particular frame, T units of time prior to the actualtransmission time of that frame. Generally, T is the worst casetransmission delay between the base station controller and the basetransceiver stations.

The worst case transmission delay between the base station controllerand the base transceiver stations can be determined by sendinginformation from the base station controller to the base transceiverstations that is time stamped. The time stamped information can becompared with common reference clock at each of the base transceiverstations to determine the worst case delay between the base stationcontroller and each of the base transceiver stations. The delayassociated with each base transceiver station can be communicated backto the base station controller so that future scheduling can include“look-ahead” scheduling. That is, the scheduler computes a schedule fora particular frame, T units of time prior to the actual transmissiontime of that frame. T can be base upon one or more transfer delay times,and can include an extra margin.

The sub-protocol data units are transferred from the base stationcontroller to the base transceiver stations through the previouslydescribed standard networking protocols. The standard network protocols(for example, ATM or IP) are generally termed “packet switched”networks. Transfer delays through a packet switched network aredependent upon the amount of packets being switched through the networkat a given point in time. That is, if the amount of packets (traffic) ofthe packet switched network is greater than usual, then the transferdelay times will be greater than usual. Therefore, the delay of thenetwork between the base controller station and the base transceiverstations can vary depending upon the level of traffic on the network.

The variable delays through the network between the base controllerstation and the base transceiver stations can be compensated for byperiodically sampling the delay times and adjusting the look ahead timeT accordingly. The look ahead time T can be set to a mean or averagevalue of the measured time delays. Additionally, an extra bit of margincan be added to the time T to make absolutely sure that the look aheadtime T is greater than the delay times between the base controllerstation and the base transceiver stations. The margin can be base upon astatistical estimation. For example, the margin can be two or threesigmas greater than a mean of several different measured delay times.

The discussion above for estimating the delay time between a basecontroller station and transmitting base transceiver stations is alsoapplicable for estimating the delay between a home base transceiverstation and transmitting base transceiver stations.

It should be understood that the look ahead scheduling is only requiredwhen transmitting simultaneously from more than one base transceiverstation to a single subscriber (receiver) unit. If communicationdiversity or spatial multiplexing is required for transmission, thenlook ahead scheduling is required because more than one base transceiverstation is transmitting to a subscriber (receiver) unit. If transmissionis between only a single base transceiver station and a singlesubscriber unit, then look ahead scheduling is not required.

Generally, there are three modes of transmission. A first mode includestransmission between a single base transceiver station and a singlesubscriber unit. This mode does not require look ahead scheduling. Asecond mode includes diversity or spatial multiplexing transmission, andrequires look ahead scheduling. A third mode includes both single basestation and multiple base transceiver station transmission. The thirdmode is useful for transmitting sub-protocol data units through a singlebase transceiver station during an initial period of transmission beforespatial multiplexing through multiple base transceiver stations can beinitiated.

Radio Frequency (RF) signals are coupled between the transmitterantennae and the receiver antennae. The RF signals are modulated withdata streams comprising the transmitted symbols. The signals transmittedfrom the transmitter antennae can be formed from different data streams(spatial multiplexing) or from one data stream (communication diversity)or both.

FIG. 4 shows another embodiment of the invention. The embodiment of FIG.4 includes a home base transceiver station 410. The home basetransceiver station 410 includes the functionality of both the basecontroller station 310 and the first base transceiver station 330 ofFIG. 3.

By combining the functionality of the base controller station and a basetransceiver station, the overall complexity of the system can be reducedbecause an interconnection between the base controller station and onebase transceiver station is eliminated. Additionally, compensation forthe delay between the base controller station and the one basetransceiver station no longer required.

An embodiment of the invention includes the home base transceiverstation being the base transceiver station that has the best qualitylink with the receiver unit. The link quality can change with time.Therefore, the base transceiver station designated as the home basetransceiver station can change with time.

Typically, the base transceiver station that has the highest qualitytransmission link with the receiver unit is scheduled to transmit thegreatest amount of information to the receiver unit. This configurationlimits the amount of sub-protocol data units that must be transferredfrom the home base transceiver station to the other base transceiverstations.

Base Transceiver Station Interface

FIG. 3 shows a base station controller that interfaces with several basetransceiver stations. FIG. 4 shows a base transceiver station thatinterfaces with several other base transceiver stations. As previouslymentioned, these network interfaces can be implemented with eitherasynchronous transmission mode (ATM) or internet protocol (IP)technology. It is to be understood that ATM and IP technologies areprovided as examples. Any packet switched network protocol can be used.

FIG. 5 shows the time delays between the base station controller 310 andthe base transceiver stations 330, 350, 370 of FIG. 3. A first timedelay t₁ indicates the time delay required for transferring sub-protocoldata units from the base station controller 310 to the first basetransceiver station 330. A second time delay t₂ indicates the time delayrequired for transferring sub-protocol data units from the base stationcontroller 310 to the second base transceiver station 350. A third timedelay t₃ indicates the time delay required for transferring sub-protocoldata units from the base station controller 310 to the third basetransceiver station 370. Generally, the time delays t₁, t₂, and t₃ arenot equal. As mentioned previously, to compensate for the variabledelays, the scheduler computes a schedule for a particular frame, Tunits of time prior to the actual transmission time of that frame.Generally, T is greater than the greatest transmission time delay t₁,t₂, and t₃.

As previously described, the variable delays through the network betweenthe base controller station and the base transceiver stations can becompensated for by periodically sampling the delay times and adjustingthe look ahead time T accordingly. The look ahead time T can be set to amean or average value of the measured time delays. Additionally, anextra bit of margin can be added to the time T to make absolutely surethat the look ahead time T is greater than the delay times between thebase controller station and the base transceiver stations. The margincan be base upon a statistical estimation. For example, the margin canbe two or three sigmas greater than a mean of several different measureddelay times.

FIG. 6 shows the time delays between the home base transceiver station410 and the base transceiver stations 450, 470 of FIG. 4. A fourth timedelay t₄ indicates the time delay required for transferring sub-protocoldata units from the home base transceiver station 410 to the basetransceiver station 450. A fifth time delay t₅ indicates the time delayrequired for transferring sub-protocol data units from home basetransceiver station 410 to the base transceiver station 470. Generally,the time delays t₄ and t₅ are not equal. As mentioned previously, tocompensate for the variable delays, the scheduler computes a schedulefor a particular frame, T units of time prior to the actual transmissiontime of that frame. Generally, T is greater than the greatesttransmission time delay t₄, t₅.

The delay associated with each base transceiver station can becommunicated back to the base station controller or home basetransceiver station so that future scheduling can include “look-ahead”scheduling. That is, the scheduler computes a schedule for a particularframe, T units of time prior to the actual transmission time of thatframe.

Sub-protocol Data Unit Encapsulation

FIG. 7 shows an embodiment of a sub-protocol data unit. The sub-protocoldata unit includes block header bytes 705, 710, header bytes 715, 720,725, payload bytes 730 and a cyclic redundancy check byte 735.

The block header bytes include a frame number byte 705 and a block, slotand mode byte 710. The frame number byte indicates the frame in whichthe sub-protocol data unit is to be transmitted. The block and slotindicate the frequency block and time slot the sub-protocol data unit isto be transmitted. The mode can be used to indicate the modulation type,coding, order of spatial multiplexing and order of diversity to be usedduring transmission of the sub-protocol data unit.

The header bytes 715, 720, 725 include header information that isnecessary for proper transmission of the sub-protocol data units. Theheader information can include identifier information, sub-protocol dataunit type information (for example, IP or ethernet packets or voice overIP), a synchronization bit for encryption, request-to-send informationfor indicating additional sub-protocol data unit are to be transmitted,end of data unit information to indicate that a present sub-protocoldata unit is a last data unit if an ethernet frame or IP packet isfragmented to one or more sub-protocol data units, and acknowledgementinformation to indicate whether sub-protocol data unit have beensuccessfully sent. It should be noted, that this list is not exhaustive.

The payload bytes 730 include the data information that is to betransmitted within the sub-protocol data units.

FIG. 8 shows a sub-protocol data unit encapsulated within an ATM cell.The basic unit of transmission of an ATM network is an ATM cell.Embodiments of the sub-protocol data units include the sub-protocol dataunits including more bytes than are included within a typical ATM cell.In this situation, the sub-protocol data unit must be segmented into twoor more pieces (depending on the size of the sub-protocol data unit). AnATM adaptation layer is required to segment the sub-protocol data unitsinto one or more ATM cells. The ATM cells can then be transmitted overan ATM network from the scheduler (base controller station or home basetransceiver station) to the base transceiver stations. Each of the basetransceiver stations receiving the ATM cell must include controlcircuitry to reconstruct the sub-protocol data units upon being receivedby the respective base transceiver stations.

A first ATM cell includes an ATM cell header 805, an adaptation header815 and an ATM payload 825 that includes a first section of asub-protocol data unit. A second ATM cell includes an ATM cell header810, an adaptation header 820 and an ATM payload 830 that includes asecond section (remaining section) of the sub-protocol data unit. ATMprotocols are well understood in the field of electronic networking.

Encapsulation of data units within smaller or larger standard data unitsis a process that is understood by those skilled in the art of networkdesign. The implementation of encapsulation processes is understood bythose skilled in the art of network design.

FIG. 9 shows a sub-protocol data unit encapsulated within an IP packet.The basic unit of transmission of an IP network is an IP packet.Generally, the IP packet comprises an IP header, a transport header 910,and a variable length payload 915. The embodiment of the sub-protocoldata unit of FIG. 5 can generally fit within the payload 915 of an IPpacket.

Reference Clock

To provide for proper timing of the transmission of the sub-protocoldata units, each of the base transceiver stations are synchronized to acommon reference clock. Generally, the reference clock can be generatedthrough the reception and processing of global positioning system (GPS)satellite signals.

Down Link Transmission

FIG. 10A shows a flow chart of steps included within an embodiment ofthe invention. A first step 1010 includes receiving the PDUs from anetwork. A second step 1020 includes creating sub-protocol data unitsfrom the PDUs. A third step 1030 includes storing the sub-protocol dataunits in sub-protocol data unit buffers. A fourth step 1040 includesestimating time delays required for transferring the sub-protocol dataunits to the base transceiver stations. A fifth step 1050 includesscheduling time slots and frequency block to each of the subscriberunits while accounting for the estimated time delays. A sixth step 1060includes transferring the sub-protocol data units from the scheduler tothe base transceiver stations. A seventh step 1070 includes transmittingthe schedule to the subscriber units. A eighth step 1080 includestransmitting the sub-protocol data units to the subscriber according tothe schedule. It is to be understood that the steps of the flow chart ofFIG. 10A are not necessarily sequential.

Up Link Transmission

FIG. 10B show another flow chart of steps included within anotherembodiment of the invention. This embodiment includes the up linktransmission procedures.

A first step 1015 includes powering up a subscriber unit.

A second step 1025 includes synchronizing the subscriber unit withframes being transmitted being transmitted from a base transceiverstation. The base transceiver station transmits information within theframes that allows the subscriber units to phase-lock or synchronizewith the base transceiver station. Generally, all base transceiverstations of a cellular system are synchronized with to a commonreference clock signal.

A third step 1025 includes decoding a map transmitted within the basetransceiver station. The transmitted map allows identification ofranging blocks and contention blocks that the subscriber can use fortransmitting information to the base transceiver station.

A fourth step 1045 includes the subscriber unit sending ranginginformation. The ranging information is sent for estimating thepropagation delay between the subscriber unit and the base transceiverstation. The estimated delay is used for ensuring that transmit timingof the subscriber unit is adjusted to compensate for the propagationdelay.

A fifth step 1055 includes decoding a map that is subsequently sent bythe base transceiver station for determining a ranging offset. Theranging offset can be used for future transmission by the subscriberunit.

A sixth step 1065 includes introducing the ranging offset in futuresubscriber unit transmissions.

A seventh step 1075 includes contending for data requests with othersubscriber units.

An eighth step 1085 includes receiving a map with block allocations inwhich data requests (up link) can be sent by the subscriber unit to thebase transceiver station.

Down Link Service Flow Request

FIG. 11A shows a set of service flow buffers 1110, 1120, 1130, 1140 thatcontain sub-protocol data units for subscriber units. The scheduler usesthe service flow buffers 1110, 1120, 1130, 1140 to generate thesub-protocol data transmission schedule. The service flow buffers cancontain different sizes of data. The scheduler addresses the serviceflow buffers, and forms the schedule.

The service flow buffers 1110, 1120, 1130, 1140 contain data for thesubscriber units. The buffers 1110, 1120, 1130, 1140 are accessible by aprocessor within the base transceiver station.

The service flow buffers 1110, 1120, 1130, 1140 can contain a variety oftypes, and amounts of data. As will be described later, these factorsinfluence how the scheduler maps the data demanded by the subscriberunits.

The scheduler accesses service flow buffers 1110, 1120, 1130, 1140,during the generation of the map of the schedule.

As depicted in FIG. 11A by arrow 1150, an embodiment of the schedulerincludes addressing each service flow sequentially and forming the mapof the schedule. As will be described later, the data blocks dedicatedto each service flow request is dependent upon a block weight. The blockweight is generally dependent upon the priority of the particular demandfor data.

Up Link Service Flow Request

FIG. 11B shows a set of estimated service flow buffer sizes 1115 1125,1135, 1145 that indicate demands for up link data by subscriber units.The scheduler uses the estimated service flow buffer sizes 1115, 1125,1135, 1145 to generate the sub-protocol data up link transmissionschedule. The scheduler addresses the estimated service flow buffersizes forming the schedule.

The estimated service flow buffer sizes 1115, 1125, 1135, 1145 areestimated demands for data by the subscriber units. The estimatedservice flow buffer sizes 1115, 1125, 1135, 1145 are generallywirelessly received from the subscriber units by the base transceiverstation. The estimated service flow buffer sizes 1115, 1125, 1135, 1145can be queued in memory buffers that are accessible by a processorwithin the base transceiver station.

As depicted in FIG. 11B by arrow 1155, an embodiment of the schedulerincludes addressing each estimated service flow buffer size sequentiallyand forming the map of the schedule. As will be described later, thedata blocks dedicated to each estimated service buffer size is dependentupon a block weight. The block weight is generally dependent upon thepriority of the particular demand for data.

A service flow request represents bi-directional requests (up stream anddown stream) between a base transceiver station and a subscriber unit,with an associated set of quality of service parameters. Examples ofservice flow requests include constant bit rate (CBR) and unrestrictedbit rate (UBR) service flow requests.

The CBR service flow requests include the scheduler scheduling thesubscribers to receive or transmit sub-protocol data units periodically.The period can be a predetermined number of times per frame. Once aservice flow request is made, the up link and down link bandwidthallocation is periodic. Information is transmitted to and from thesubscriber units without the subscriber units having to send informationsize requests. Up link allocations are periodically scheduled withoutsolicitation by the subscriber unit.

The UBR service flow requests include the scheduler scheduling the uplink and down link scheduling based upon information size requests bythe subscribers. The down link map allocations are made based upon theamount of data in the associated service flow buffers. The up link mapallocations are made based upon the information size requests sent bythe subscriber units. Each information size request updates thescheduler estimate of the amount of data in an associated service flowbuffer.

Orthogonal Frequency Division Multiplexing (OFDM) Modulation

Frequency division multiplexing systems include dividing the availablefrequency bandwidth into multiple data carriers. OFDM systems includemultiple carriers (or tones) that divide transmitted data across theavailable frequency spectrum. In OFDM systems, each tone is consideredto be orthogonal (independent or unrelated) to the adjacent tones. OFDMsystems use bursts of data, each burst of a duration of time that ismuch greater than the delay spread to minimize the effect of ISI causedby delay spread. Data is transmitted in bursts, and each burst consistsof a cyclic prefix followed by data symbols, and/or data symbolsfollowed by a cyclic suffix.

FIG. 12 shows a frequency spectrum of OFDM sub-carrier signals 1210,1220, 1230, 1240, 1250, 1260. Each sub-carrier 1210, 1220, 1230, 1240,1250, 1260 is modulated by separate symbols or combinations of symbols.

An example OFDM signal occupying 6 MHz is made up of 1224 individualcarriers (or tones), each carrying a single QAM symbol per burst. Acyclic prefix or cyclic suffix is used to absorb transients fromprevious bursts caused by multipath signals. Additionally, the cyclicprefix or cyclic suffix causes the transmit OFDM waveform to lookperiodic. In general, by the time the cyclic prefix is over, theresulting waveform created by the combining multipath signals is not afunction of any samples from the previous burst. Therefore, no ISIoccurs. The cyclic prefix must be greater than the delay spread of themultipath signals.

Frame Structure

FIG. 13A shows a frame structure depicting blocks of transmission datadefined by transmission time slots and transmission frequency blocks.The scheduler maps requests to transmit or receive data into such aframe structure. For example, data blocks B1, B2 and B3 can betransmitted during a first time slot, but over different frequencyranges or blocks. Data blocks B4, B5 and B6 are transmitted during asecond time slot, but over different frequency ranges or blocks thaneach other. The different frequency ranges can be defined as differentgroupings or sets of the above-described OFDM symbols. As depicted inFIG. 13A, the entire transmission frequency range includes threefrequency blocks within a frame.

Data blocks B1, B6, B7, B12, B13, B18, B19, B24, B25 and B30 aretransmitted over common frequency ranges, but within different timeslots. As depicted in FIG. 13A, ten time slots are included within asingle frame. The number of time slots per frame is not necessarilyfixed.

The numbering of the data blocks is depicted in the order chosen becauseof ease of implementation.

The data blocks generally occupy a predetermined amount of frequencyspectrum and a predetermined amount of time. However, due to thevariations in the possible types of modulation, the number of bitstransmitted within a block is variable. That is, typically the datablocks include a predetermined number of OFDM symbols. The number ofbits within an OFDM symbol is based on the type of modulation used intransmission. That is, a 4 QAM symbol includes fewer bits than a 16 QAMsymbol. The number of bits included within a sub-protocol data unit isgenerally set to a predetermined number. Additionally, depending uponthe quality of the transmission link, the bits to be transmitted can becoded, adding additional bits. Therefore, the number of sub-protocoldata units included within a data block is variable. The variability ofthe number of sub-protocol unit included within a data block will bediscussed further when discussing the transmission modes.

FIG. 13B shows two maps 1310, 1320. A first map 1310 can be designatedas the up link map, and a second map 1320 can be designated as the downlink map. As shown in FIG. 13B, the up link map 1310 occupies adifferent frequency band than the down link map 1320. As describedbefore, the maps include a finite number of frequency blocks and timeslots. The maps 1310, 1320 of FIG. 13B are consistent with FDDtransmission.

FIG. 13C also shows two maps 1330, 1340. A first map 1330 can bedesignated as the up link map, and a second map 1340 can be designatedas the down link map. As shown in FIG. 13C, the up link map 1330occupies a different time duration than the down link map 1340. Asdescribed before, the maps include a finite number of frequency blocksand time slots. The maps 1330, 1340 of FIG. 13C are consistent with TDDtransmission.

Service Flow Request Table

FIG. 14 shows an example of a service flow table. The service flow tabledepicts information about each service flow request that is useful ingenerating the data block transmission schedule. The informationincluded within the service flow table includes a service flow requestidentification number (SF₁, SF₂, SF₃, SF_(N)), a service flow queue size(SFQ₁, SFQ₂, SFQ₃, SFQ_(N)), a mode assignment (M₁, M₂, M₃, M_(N)) ablock weight (BW₁, BW₂, BW₃, BW_(N)), and system mode (SM (spatialmultiplexing), Diversity).

The service flow request identification number identifies eachindividual service flow request.

The service flow queue size provides information regarding the size oramount of information being requested by the service flow request.

The mode assignment provides information regarding the type ofmodulation and coding to be used when providing the data blocks of theservice flow request. The mode assignment is generally determined byquality of the transmission link between the base station transceiverand the subscriber units. The quality of the transmission link can bedetermined in many different ways.

The transmission quality of the links between a subscriber unit and thebase transceiver stations can be determined several different ways. Acyclic redundancy check (CRC) failure rate can be monitored. The higherthe quality of the link, the lower the CRC failure rates. The monitoringof CRC failure rates of steams of symbols is well known in the field ofcommunications.

A signal to interference of noise ratio (SINR) monitoring can also beused to determine the quality of the transmission links. Varioustechniques as are well known in the field of communications can be usedto determine the SINR.

Based on the quality of the link between a base station transceiver anda subscriber unit, a transmission mode is assigned to the subscriberunit. As previously mentioned, the transmission mode determines thecoding and modulation used in the transmission of data between the basestation transceiver and a subscriber unit. The better the quality of thetransmission link, the greater the amount of information that can betransmitted. For example, the better the quality of the link, thegreater the allowable order of modulation. That is, 16 QAM generallyrequires a better transmission link than 4 QAM.

A poor quality link can require the transmitted data to be coded tominimize the error rate of the transmitted data. Generally, coding ofthe transmitted information reduces the rate the data is transmittedbecause the coding adds additional coding data. Examples of the types ofcoding used include convolutional coding and Reed Solomen coding. Thesecommon types of coding are well known in the field of communications.

The mode assignment can also determine other transmissioncharacteristics. For example, the mode assignment can also be used forspecifying transmission frequency bandwidth or transmission power.

The block weight determines the minimum number of previously describedblocks that are allocated to a service flow request at a time. The blockweight is generally determined according to the priority of the databeing requested. That is, certain types of service flow requests are forhigher priority information. By allocating a larger block weight, theservice flow request will be satisfied more quickly.

For a service request having a block weight of two, for example, themapping of the schedule will allocate two successive blocks to theservice request. A larger block weight will cause a larger number ofblocks to be allocated to a service request.

The system mode determines whether the transmission of the data includesspatial multiplexing, diversity, or neither. Again, the quality of thetransmission link between the base station transceiver and thesubscriber units generally determines whether the transmission shouldinclude spatial multiplexing or diversity.

FIG. 15 shows a flow chart of steps included within an embodiment of ascheduler according to the invention. As previously mentioned, thescheduler assigns time slots and frequency blocks in which sub-protocoldata units are to be received by particular subscriber units. A scheduleis generated once per a frame unit of time. A predetermined number ofdata blocks are included within a frame. Generally, the schedulerincludes a weighted round robin assignment methodology.

The scheduler is generally implemented in software that runs on thecontroller within the base transceiver station. The controller isgenerally electronically connected to the MAC adaptation unit, thesub-protocol data buffers and the framing unit.

A first step 1510 includes addressing a service flow request.

A second step 1520 includes whether the present service flow requestincludes data to be sent. If data is to be sent, then the schedulerassigns the present service flow request to one or more data blocksbased on the mode, block weight and system mode.

A third step 1530 includes updating the service flow queue. That is,sub-protocol data units have been assigned to data blocks, then theservice flow queue should be updated to reflect the assignment.

A fourth step 1540 includes incrementing a block count. As previouslymentioned, the mapping of a schedule only occurs once per frame. Eachframe generally includes a predetermined number of frequency blocks andtime slots. The block count begins when creating a map of a schedule. Asservice flow requests are addressed, a block counter is incremented.Note that the block weight will factor into the block count.

A fifth step 1550 includes checking whether the block count is equal tothe predetermined number N. If the block count has reached thepredetermined number, then all of the blocks within the present framehave been assigned. If the block count is less than the predeterminednumber N, then more blocks within the frame can be assigned sub-protocoldata units.

A sixth step is executed once all of the blocks within a frame have beenassigned. The mapped schedule of the frame can then be sent.

Transmission Modes

FIG. 16 depicts several modes of block transmission according to theinvention. The mode selection is generally based upon the quality of thetransmission link between the base station transceiver and thesubscriber units. The mode selection can determine the type ofmodulation (for example, 4 QAM, 16 QAM or 64 QAM), the type of coding(convolution or Reed Solomon), or whether the transmission includesspatial multiplexing or diversity.

As previously stated, several transmission link parameters can be usedto establish the mode associated with the transmission of a sub-protocoldata unit requested by a service flow. FIG. 16 depicts a relationshipbetween a transmission data block (defined by a frequency block and timeslot) and sub-protocol data unit according to an embodiment of theinvention.

FIG. 16 shows a single time slot that is divided into three data blockfor six different modes. A first mode 1610 includes a sub-protocol dataunit occupying two data blocks. A second mode 1620 includes asub-protocol data unit occupying a single data block. A third mode 1630includes three sub-protocol data units occupying two data blocks. Afourth mode 1640 includes two sub-protocol data units occupying one datablock. A fifth mode 1650 includes five sub-protocol data units occupyingtwo data blocks. A sixth mode 1660 includes three sub-protocol dataunits occupying a single data block.

As previously stated, the mode assignment determines the amount ofinformation transmitted within each data block. Generally, the betterthe quality of the transmission link between a base transceiver stationand a subscriber unit, the higher the mode assignment, and the greaterthe amount of information transmitted per data block.

It should be understood that the mode assignment of transmission linksbetween base transceiver stations and subscriber units can vary fromsubscriber unit to subscriber unit. It should also be understood thatthe mode assignment of a transmission link between a base transceiverstation and a subscriber unit can change from time frame to time frame.

It is to be understood that the number of frequency blocks allocated pertime slot is variable. An embodiment of the scheduler includes thescheduler taking into consideration constraints on the frequencybandwidth on either the up link or the down link transmission. Thefrequency bandwidth allocations can be adjusted by varying the number offrequency blocks within a time slot. The frequency bandwidth allocatedto a subscriber can be limited due to signal to noise issues, or theFederal Communication Committee (FCC) limitations. The scheduler canaccount for these limitations though allocations of frequency bandwidththrough the scheduling.

The description of the invention has been limited to FDMA and TDMA.However, it is to be understood that the principles and concepts of theinvention can be extended to include CDMA.

Sleep and Paging Modes

The subscriber units can be configured to include a sleep or pagingmode. In the sleep mode, the subscriber units that are not scheduled toreceive or transmit data units, power down to save power. That is, ifthe map schedule of a frame does not include transmission between anybase transceiver station and a subscriber unit, the subscriber unitpowers down for that particular frame. Therefore, the subscriber unitrequires less power. A paging mode extends the power down period tomultiple frames. In paging mode, a subscriber unit only powers up when arequest for transmission of data is received. The request can bereceived at particular points in time, for example, when synchronizationsignals are received by the subscribers from the base transceiverstations.

Data Block Headers

As previously mentioned, the map of the schedule of each frame istransmitted to all subscriber units at the beginning of the transmissionof a frame. Additionally, the service flow identification and modeselection for each frequency block and time slot is generallytransmitted within a header of the data block transmitted within thefrequency block and time slot.

FIG. 17 shows a structure of a map message that is sent once per frame.The map message includes a header 1705, and information elements (IE's)1710, 1720, 1730, 1740. The header includes the number of the associatedframe. The IE's 1710, 1720, 1730, 1740 include a service flowidentification, a mode number, the number of blocks associated with theservice flow identification, and information indicating whether theservice flow is up link or down link.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The invention islimited only by the appended claims.

1. A wireless communication device comprising: a controller running ascheduler, to generate a schedule of time slots and frequency blocks inwhich sub-protocol data units are to be transmitted from one or moretransmitter(s) to the subscriber unit based, at least in part, on anestimate of time delays incurred in communicating sub-protocol dataunits from the scheduler unit to the transmitter(s).
 2. A wirelesscommunication device according to claim 1, further comprising: one ormore transmitter(s), responsive to the scheduler, to transmit at least asubset of sub-protocol data units to one or more remote communicationdevice(s).
 3. A wireless communication device according to claim 1,wherein the time delays are used to generate the schedule by using thetime delays to project a timing of when the sub-protocol data units areto be wirelessly transmitted from the transmitters.
 4. A wirelesscommunication device according to claim 3, wherein the time delay areused to generate a look ahead schedule that compensates for the timingdelays of the sub-protocol data units from the scheduler unit to thetransmitters.
 5. A system comprising: one or more antenna(e) throughwhich a wireless communication channel can be established with a remotesystem; one or more transmitters, selectively coupled with the one ormore antenna(e) to transmit content including sub-protocol data units toremote communication device(s); and a controller running a scheduler,selectively coupled to at least a subset of the one or moretransmitters, to generate a schedule of time slots and frequency blocksin which sub-protocol data units are to be transmitted from selects onesof the one or more transmitter(s) based, at least in part, on anestimate of time delays incurred in communicating sub-protocol dataunits from the scheduler unit to the transmitter(s).